The production or recovery of crude oil or bitumen from viscous substances depends from the point of view of efficiency on the praticular method used. In the instance of withdrawal of crude oil from a reservoir, the producing technique is relatively inexpensive in contrast to the producing of bitumen from a substrate comprising tar sand and similar bitumen-bearing substances. On a trial basis the producing of tar sands to achieve any desirable output of bitumen, far exceeds the cost of producing crude oil.
However, because of the known limited crude oil sources and reservoirs it has become necessary to use diligence toward developing every known method for producing tar sand sources. Basically it is known that a certain quantity of bitumen can be obtained through various processes depending on the character of the substrate or the source from which the bitumen is obtained.
Among the most prevalent means for achieving such production is through stimulation. In this process steam is among the most extensively used recovery media. In different procedures, steam has even been successful in producing oil at commercial rates where pay is thick, viscosities at reservoir conditions are low, and some reservoir energy exists.
By way of example, in one embodiment of a steam stimulation process, high pressure steam containing natural gas, is forced into an unconsolidated substrate at a pressure sufficient to part or break up the deposit. In effect, the injected steam heats and fractures the deposit; it thereby creates an irregular heated area around the injection well. Following said injection heating period, which normally lasts up to several weeks, the well is put on production. Heated bitumen then moves through the substrate and into the producing well.
In another embodiment of the producing technique, the oil sands are underlain by a highly permeable zone. It has been proposed in such instance that steam be injected into this zone. Following the achievement of communication, pressure within the zone is built up and depleted in a number of cycles. This sequence causes the overlying bitumen to be heated and driven to the production well by a combination of gravity drainage and pressure depletion gradients.
It is clear then, that because of the varied composition of the substrate and tar sands within which the bitumen is contained, a particular injection process is preferably tailored to meet specific substrate conditions. Among the most pertinent variable characteristics, viscosity of the flowable material is primary. Viscosity measurements are available for example for the temperature range of from 60.degree. to 250.degree. F. Some extrapolation can be made into lower and higher ranges. However, no data is available to suggest viscosity in the temperature ranges up to 700.degree. F. which are important for in-situ recovery operations.
Another factor to be considered in assessing a substrate is its permeability. A fundamental parameter such as permeability of the substrate, must be known as a function of bitumen concentration as well as of grain size distribution. Also desirable is a knowledge of how the various minerals are arranged in the formation. For example, if bitumen occupies 100% of the void space then at reservoir conditions the formation is impermeable. If, however, water occupies 100% of the void space then obviously many oil and tar sands are highly permeable.
In many recovery techniques and operations, a complex set of chemical reactions takes place. For example, it is known that air reacts with bitumen at room temperature, and in varying degrees of temperature right up to the combustion temperature of carbon. Such knowledge aids in evaluating air requirements for combustion operations. It further indicates the desirability of controlling the oxidation temperature. Even without the presence of oxygen, bitumen undergoes a complex set of thermal changes which eventually lead to the deposition of coke.
It is known further that many hydrocarbons precipitate asphaltenes from bitumen. However, little knowledge is available on the critical pressure, temperature, or concentration ranges in which asphaltenes are fully or partially precipitated. The degree, nature, and extent of such precipitation is essential information in planning any in-situ recovery operation.
The interaction within a substrate of injected materials with the mineral matrix at the temperatures and pressures used in many recovery operations, can result in severe changes to the matrix. This will result in the solution and reprecipitation of mineral components and/or changes of some minerals into others. Examples of this are the solution and reprecipitation of quartz, or the accelerated formation of montmorillonite. Such effects can adversely affect flow patterns, and therefore require further study.
To more completely bring together the above reiterated elements and variables, it is desirable to determine the characteristics of a subterranean area to be produced. This is done most readily by the taking of samples. These normally take the form of core samples, which are extracted directly from the substrate through the use of special core-taking equipment.
Toward expediting the producing operation, it is further desirable to perform as many tests as possible. This is particularly important prior to the commencement of large scale operations in a particular field. Normally it is achieved by bringing a segment of the formation, such as for example a core or a number of cores, a mineral block, or reconstituted oil sands, into a controlled environment. In the latter, the sample unit is subjected to a similar set of conditions as are found in the actual field. Under the simulated conditions, in-situ recovery concepts can be tested in a rapid and less costly manner.
It is therefore a primary object of the invention to provide a method and the apparatus therefore which will permit core samples of a tar sand and similar bitumen-bearing substrate to be evaluated. Such evaluation will thus provide data and information necessary to determine which of the areas from which the cores were taken best respond to a specified recovery process and thus determine the location to produce on a large scale whereby to obtain the most efficient yield of bitumen from the field.
In brief, the disclosed method for evaluating individual subterranean samples or specimens, comprises a sequence of controlled steps. Primarily, core samples withdrawn from a particular field or area are subjected to one or more steps as required, to simulate a predetermined bitumen extraction process. The primary step is the heating of the core specimen by injection and withdrawal of the fluids from one end of said core. This is achieved through use of a medium such as steam which is injected in the desired amount, and for a particular time period, into the core specimen. As the bitumen component contained within the specimen is elevated in temperature, it separates therefrom either by gravity drainage or induced pressure gradients. Such pressure gradients may be induced by agents used and/or periodically reducing the injection pressure and/or alternating injection periods with production periods.
Thereafter by comparing the temperature and pressure response at the top of the core, the volume of bitumen obtained, and by further evaluating the character of the residual sand in the residual core sample, it is possible to determine the character of the response of different areas as an aid to field site selection.